Enable PyBytes in Micropython build

I've built the PyCom Micropython image for LoPy from scratch following https://github.com/pycom/pycom-micropython-sigfox and flashed it. The flash overwrote pybytes_config.json, so I added that again and rebooted the device, but PyBytes never starts. It looks like you can only get PyBytes through the updater, or downloading the board image from the PyBytes console. Is there any way to get PyBytes to run on boot with a custom Micropython build?